Hmm. I'm not sure. It can be different from state to state. However, the purpose of 'serving' papers is to give the person notice that a lawsuit or such legal action has been filed against said person.
Now, the process server apparently had the fiance's address, and the process server will certify that he/she served the papers on a person residing within said residence.
Oddly enough, there are many people whom do not wish to be served with a lawsuit or other legal demand. Thus, even though they are at home, they will not answer the door, or have some other person within the house answer for them, and claim they are not there. Hence many jurisdictions allow the process server to leave the papers with whoever happens to be in the house.
Now, the fiance may choose to ignore the papers, at his peril. He then risks a default judgement (if it is that type of case) or be found to be in contempt of court for not appearing at a hearing (if that is what the papers are about). Fiance may then go to court and 'contest' the judgement or contempt order, claiming that he was not personally served with said papers and had no idea that he was involved in a legal proceeding. However, he may find out that in your jurisdiction 'actual service' is not required, only 'presumed service' (that is not the precise term, but I can't think of it right now), meaning that since it was his (fiance's) legal residence, and someone within that residence was served the papers, then it is reasonable to presume that the person that accepted the papers (willingly or not) gave the papers to the person.
My opinion: it is not worth the risk to just ignore the process. I imagine that said fiance now has actual knowledge that these papers exist, and he may as well face up to whatever he is facing.
By the by: I do think that in criminal cases the standard is a bit more strict (i.e., actually giving the papers to the person in question).